The present invention relates to a sewing machine with a cloth-edge detector for detecting a border of the overlap of cloths or an edge of a single cloth.
In one prior-art cloth-edge detector, the sensitivity in detecting a cloth-edge is adjusted with a manually operated controller. As shown in FIG. 8, the detector has a photo-transistor P1 that applies a voltage according to the intensity of light it receives through a cloth or cloths to a negative terminal of a comparator C1 via a filter circuit F1 and an amplifier A1. The voltage at the negative terminal is compared with a preset reference voltage Vref at the positive terminal in the comparator C1 to distinguish the number of cloths. Before sewing, the collector current that flows through the photo-transistor P1 is adjusted with a manually controlled variable resistor R1, so that the detector detects a cloth-edge by distinguishing a thin part from a thick part while sewing the cloths.
It is, however, difficult and inconvenient to adjust the variable resistor R1 by hand. To remove this disadvantage, Japanese Published Unexamined Patent Application No. S60-85386 and S58-50487 discloses a sewing machine with an automatically adjusted cloth-edge detector. As shown in FIG. 9, this cloth-edge detector has a variable-voltage generator VR1 (or a digital-to-analogue converter) controlled by a microcomputer. A voltage V2 (FIG. 7) for one cloth is first detected. Then, the reference voltage V6 is set slightly lower than the voltage V2, and the variable-voltage generator VR1 applies the reference voltage V6 to the positive terminal of a comparator C2. While sewing, the cloth-edge detector distinguishes the number of cloths to detect a cloth-edge. In FIG. 9, the photo-transistor P2, filter circuit F2, and amplifier A2 are similar to those illustrated in FIG. 8, and a resistor R2 determines the collector current.
Such an automatically adjusted cloth-edge detector still has problems. Because the reference voltage V6 is set lower than the voltage V2 by a small amount, the reference voltage V6 may be set too close to the voltage V2 for a single cloth or too far from the voltage V1 for the overlapping cloths. Moreover, an uneven weave of a cloth could cause the irregularity in the strength of the light that reaches the light receiver past the cloth. If the reference voltage V6 is set for the light passing through a loose weave of the cloth, the detector would mistake a tight weave as an overlap of the cloths. The prior-art detector, therefore, might misjudge a cloth-edge when the weave of the cloth is uneven.